Wednesday, April 9, 2008

Mounded Plantings, Irrigation-Free

I’ve started the annual ritual of mulching the perimeter of my irrigation-free planted mound. I lay down 5 or so sheets of newspaper and then several inches of compost made of rice hulls and manure.

I've always wondered why the former Xeriscape Council only advocated a savings of up to 75% on the water used for outdoor irrigation. Why only 75%? Why not shoot for 100%? After seven years of trial and error, I came up with a mound-and-plant system some eight years ago which has required no additional irrigation beyond the day of planting, even during the most protracted California drought in over 100 years.

To begin such a planting scheme, I stockpiled chunky wood chips (not sawdust, which settles down too much and doesn't drain well) from local tree-trimming services. This is critical that the wood chips are mixed with the fresh green, fresh leaves, and small branches. The leafy green parts are critical to compensate for the carbon in the woodier chips.

Next, I used a spading to just crack open the soil beneath the planned area of the mound, not heaving it. Then, I built an active compost pile and plant directly on top of trimmings. Because tree chips are so high in carbon, I layered or mixed them with some manure if required—if there were not enough leaves. I'd guesstimate that a good starting ratio for your own experiments would be one part manure to three or four parts chips. I would use other types of high-nitrogen materials if needed—wet kitchen scraps, fresh grass clippings, green-manure crops like buckwheat, vetch, bell beans, and clover—to help decompose the woody chips. The more nitrogen added, the faster the mound will decompose and the greater the nitrogen supply for the growing plants. I piled this mixture of high-carbon and high-nitrogen materials at least 50% higher than I wanted the final mound to be, and sometimes up to double the height. I watered each layer as I went and made sure all material was moist.

Next, I covered the mound with a soil cap at least eight inches thick—the thicker, the better. I used a mixture of 50% rotted turkey bedding and 50% native soil for the top layer. The soil cap insures good drainage, a neutral soil temperature, balanced nutrition, and good initial growth for the transplants. The plants are placed into the soil cap and watered in thoroughly. I used five sheets of overlapping newspapers covered by turkey-bedding mulch cover the soil cap and, like a "biodegradable herbicide," suppress all weed seedlings. It’s best for me to plant in the fall so the winter rains continue to keep the composting heap moist and to allow the roots to grow. Drought-resistant plants are essential. I used white, pink, and blue rosemary, grey and green santolinas, rhue, many types of lavender, native sages, euphorbia, daffodils, and society garlic (which the deer started eating a few years later).

Once the mound started rotting, the root-hairs of the plants followed the decomposition to take advantage of the newly available nutrients. Plants are "smarter" than we often acknowledge; their roots won't grow into areas that are too warm due to thermophilic (hot) decomposition over 110 F.

Eventually, my "research" mounds settled down, the shrubs rooted fully into the native soil—the new mound is a wonderful, curvilinear feature in my landscape. Mounded plantings, which seemed to me to be a heap of contradiction at first, have become one of my preferred techniques for quick no-till soil drainage, and they don't require any drip-irrigation hardware or precious water. In hotter climates a bit of drip irrigation will probably be needed. If you experiment with this, please let me know.

There's more about planting on mounds in my book Roots Demystified.

NOTE: I live in a moderate-summer climate with a moderating marine influence. If you attempt this mound-and-plant strategy in your own area, I suspect you'll have to make some changes in plant selection and the time of year you’ll be planting. For example, I once installed a test plot at Kit Anderson’s house near Burlington, VT, when she was editor of National Gardening Association magazine. I spent hours in the muggy August heat, along with members of the NGA staff, hauling in manure-rich straw, distributing Kit's garden clippings and leaves and planting a range of perennials which normally thrive in this northern latitude (within one-half mile of Lake Champlain). Many of the crew were skeptical that such a bizarre method would work, and it didn’t—growth in the late summer was not sufficient to allow many of the perennials to weather the winter in good form. I suspect that timing was the main problem, and that mounds in hot-summer areas need to be planted in early spring so the roots can be deep enough by fall to tolerate the frozen months. You'll have to experiment in your soil and climate. I suspect the mound should be started in the Spring were there is summer rain.

Please post a comment - I want to know how the mound system worked for you.

To see more detail of the illustration put you cursor over the image and double click.

All rights reserved, Copyright 2008

Visit my web site to learn about my gardening books.

NOTE: The comments section at the bottom of the post has disappeared. Click on the "___ Comments" button or the title under the "Blog Archives". Thanks, Robert

No comments: